1. Field of the Invention
The present invention relates in general to frequency modulation measurement and more particularly to a novel method and apparatus for accurately testing and adjusting the modulation sensitivity and, hence, the frequency deviation of frequency modulated radio frequency generators.
2. Description of the Prior Art
A technique for measuring the frequency deviation of a frequency-modulated generator is disclosed in U.S. Pat. No. 3,071,726, issued Jan. 1, 1963, inventor B. E. Nelson, in which the Bessel zero or carrier null technique is disclosed. This technique is based on the fact that the RF carrier voltage amplitude goes to zero when the frequency modulation index equals 2.405. The modulation index is defined as M.sub.f = .DELTA.f/f.sub.s where .DELTA.f is the peak frequency deviation; and f.sub.s is the modulation frequency. At the carrier null condition .DELTA.f/f.sub.s = 2.405. The amplitude of the modulation frequency is increased from zero amplitude to an amplitude at which the carrier shrinks to zero amplitude thereby making the modulation index M.sub.f equal to 2.405. By measuring the amplitude V.sub.s of the modulation frequency f.sub.s at the carrier null condition, the modulation sensitivity .DELTA.f/V.sub.s of the frequency modulator is determined.
Such a technique, as is disclosed in U.S. Pat. No. 3,071,726, requires the use of a modulation frequency having an adjustable amplitude so as to drive the RF carrier voltage to zero and, also, requires some device for measuring the RF carrier output voltage during the measurement period, thus, separate expensive test equipment is required to implement this technique.
Another frequency deviation measuring system is disclosed in U.S. Pat. No. 3,549,997, issued Dec. 22, 1970, inventor D. Rotzel, wherein a frequency modulated signal and a reference signal are separately used to derive zero crossing pulses which are separately coupled to counters via gates. The frequency modulated signal is additionally coupled to a pulse shaper via a discriminator which generates a pulse which opens the gates to initiate counting. Each gate is separately closed by its associated counter when a predetermined count is reached for that counter. The time elapsing between the closing of the respective gates is a measure of the frequency deviation. The time is measured by coupling the gate closing signals via an Exclusive-OR gate to another gate which allows pulses from a timing pulse generator to reach a direct reading counter. One problem with this technique is that the time measurement will vary depending upon whether the frequency modulated signal is started during its positive half-cycle when its frequency will be greater than that of the reference frequency, or whether it is started during the negative half-cycle when its frequency will be less than the reference frequency. In order to reduce the effects of the short time fluctuations, the deviation must be determined by taking an average value of the measurements.